The present invention relates to the production of surface-modified polyethylene terephthalate materials which are desirably adapted for use as reinforcing substrates in rubber-based products.
As discussed in U.S. Pat. Nos. 3,419,463; 3,729,336; and 3,755,165 regarding the manufacture of rubber goods requiring cord reinforcement, such as in pneumatic tires, friction belts and the like, the strength and durability of the adhesive bond between the reinforcing cord and the rubber being reinforced is important. The reinforcing cord serves to increase the tensile strength of the rubber-based goods and restrict the degree of distortion during use. The reinforcing cord is normally placed within the rubber mass and must be firmly bonded to the rubber to provide a unitary structure. Stress will occur at the interface between the reinforcing element and the rubber during use. Accordingly, good bond strength and adhesion is needed to prevent separation at the interface.
Polyester reinforcing materials are increasingly being employed in the reinforcement of such rubber-based goods, and particularly as tire cord, in the manufacture of rubber tires. These polyester reinforcing materials (e.g., polyethylene terephthalate filament or cord) possess physical characteristics such as high strength, flex resistance, high stretching modulus and low creep which make them outstanding materials for such reinforcement. However, the use of polyester reinforcing materials in rubber reinforcement applications is subject to certain problems due to the poor rubber adhesion properties associated with unmodified polyester materials.
The use of aqueous dispersions containing a combination of a resorcinol-formaldehyde condensate and an elastomer latex (referred to as an RFL dip) is best known as an effective treating agent for improving the rubber adhesion of some synthetic polymer reinforcing materials such as nylon. However, the RFL dips do not satisfactorily overcome the problems encountered with the adhesion of unmodified polyester reinforcing materials due to the low level of reactive functional groups in such materials (e.g., polyethylene terephthalate).
Attempts have been made to find new treating agents to improve the rubber adhesion of polyester reinforcing materials or, in the alternative, to treat the polyester with additional agents in combination with the RFL dip treatment to improve the adhesion For example, U.S. Pat. No. 3,383,242 discloses pretreatment of polyethylene terephthalate yarn by use of an aqueous dispersion of a curable combination of a diglycidic ether of an aliphatic diol and an amine curing agent and a water-dispersible lubricating agent. U.S. Pat. No. 3,755,165 discloses a tire yarn finish composition consisting of a lubricating oil, an antistatic agent, an emulsifier, water and as an adhesive promoting agent an aliphatic diisocyanate. U.S. Pat. No. 3,834,934 discloses an adhesive composition for fibrous material comprising an admixture of RFL and a triallyl cyanurate-resorcinol-formaldehyde reaction product. U.S. Pat. No. 3,729,336 discloses treatment of polyester fibers with a composition of a copolymer comprising crystallizable ester units identical to those in the polyester fiber, poly-oxyalkylene groups and polyepoxy containing groups and a polyepoxide followed by application of RFL. U.S. Pat. No. 4,031,288 discloses pretreatment of tire cord with a solvent solution of a polyisocyanate followed by application of an adhesive such as RFL. U.S. Pat. No. 4,187,349 discloses an adhesive system for polyester tire cord comprising a heat-curable admixture of a polyepoxide and a linear carbon-to-carbon addition polymer containing pendent aminimide and N-pyrrolidonyl groups.
U.S. Pat. Nos. 2,905,582; 3,278,333; 3,547,764; 3,642,553; 3,647,513; and 3,939,294 each disclose the use of isocyanate-based adhesives to enhance the adhesion of polyethylene materials to rubber. U.S. Pat. Nos. 3,308,007; 3,547,764; and 3,803,035 each disclose the use of epoxy-based adhesives to enhance the adhesion of polyethylene materials to rubber.
U.S. Pat. No. 3,111,424 discloses a method whereby an irradiated polymer substrate (such as polyethylene terephthalate) is coated with a chemically dissimilar coating comprised of an ethylenically unsaturated compound. Suitable irradiation means include a resonant cavity accelerator, Van der Graaff generator, betatron, synchrotron, cyclotron, etc. U.S. Pat. No. 3,188,229 discloses a process for adhering an organic material on a dissimilar organic polymer substrate such as polyethylene terephthalate using high energy ionizing radiation. European Patent Application No. 43,410 discloses a process for producing an adhesive active polyester yarn wherein the yarn is initially exposed to ultraviolet irradiation and subsequently contacted with an epoxy silane-containing solution. U.S. Pat. No. 4,328,324 discloses a process for treating aromatic polyamide fibers to improve the adhesion thereof to epoxy resins and rubber wherein the fibers are passed through a plasma effected by means of a high frequency electrical field.
Although many of these prior art treatments increase the adhesion between the polyester and the rubber, in the case of polyethylene materials having a reduced carboxyl group content to improve the hydrolytic stability of the yarn, the adhesion to rubber is in many cases insufficient especially if reduced curing temperatures for the resorcinol-formaldehyde-latex RFL are used.